Frequency modulation signal detector



Feb. 8, 1944.

W. R. KOCH FREQUENCY MODULATION SIGNAL DETECTOR Fild July 31, 1941 VVvvvv PatentedFeb. 8, 1944 I FREQUENCY MODULATION SIGNAL DETECTOR Winfield R. Koch, Haddonfield, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application July 3i, 1941, Serial No. 104,901 7 Claims. (Cl. 250-27) This invention relates to frequency modulation signal detectors, and has for its primary object to provide an improved detector circuit for frequenc modulated signals for use in battery type receivers and the like, whereby the cathodes of the tube elements thereof may be operated at substantially ground or chassis potential.

It is a further object of this invention to provide an improved balanced detector for frequency modulated signals which permits the use therein of filament type tubes or cathode type tubes with the cathodes operating at substantially ground potential.

In the usual type of balanced frequency discriminator and detector circuits for frequency modulation reception, in which'amplitude variations are balanced, one or more of the cathodes of the tube involved in the circuit are required to operate at an audio frequency potential with respect to ground. This prohibits the use of filament orbattery type tubes unless a separate battery is provided to 'heat the filaments of the tubes. Furthermore, in the case of equi-potential or heater type cathodes, noise voltages may be introduced into the circuits through leakage between the heater and the cathode of each tube whichoperates with the cathode above ground potential.

In accordance with the invention, a pair of diode rectifiers are connected to a suitable frequency discriminator network and provided with a balanced output circuit in which is interposed on one side a gain adjusting circuit and an amplifier or phase inverter. With this arrangement, the phase inverter and diode rectifiers may be contained in the same envelope and provided with a common grounded cathode or filament.

The output tube or audio frequency amplifier stage following the detector output circuit may also be of the grounded cathode or filament type, thereby eliminating the necessity for utilizing separate batteries for filament excitation and preventing the introduction of noise voltages into the circuit in case heater type cathodes are employed.

The invention will however be further understood from the following description when considered in connection with the accompanying drawing, and' its scope is pointed out in the ap pended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of a frequency modulation signal detector system embodying the invention; and

Figure 21s a similar schematic circuit diagram showing a modification of the circuit of Figure 1.

Referring to Fig. 1, 5 is an intermediate frequency amplifier tube ofa frequency modulation signal receiving system having an output circuit 6 including the primary windings I and 8 of a frequency discriminator network comprising a tuned secondary circuit 9 coupled to the winding 1 and a tuned secondary circuit l0 coupled to the winding 8.

The circuits 9 and iii, respectively, are tuned slightly above and slightly below the mean intermediate frequency, and the high potential side of each circuit is connected to a diode rectifier anode, as indicated at H and I2, respectively.

The diode anodes I I and H are associated with a common cathode l3 which is connected to ground, as indicated at It, and through ground with the grounded terminals l5 and MS of diode output resistors l8 and I9, which are connected with the low potential terminals 20 and 2| of the circuits 9 and t0, respectively.

With this arrangement, the circuits 9 and I0 together with the diode rectifiers ll-i2-l3 provide two separate diode rectifier circuits in the discriminator network from which signal potentials maybe derived and appliedto a balanced output circuit comprising an impedance or resistor and an impedance Or resistor 26, serially,

. with a center tap 21 between the resistors from which signals are taken through an output circuit 28 and applied to a utilization device, such as an audio frequency amplifier stage including the tube 29. The latter is provided with a grounded cathode 30 of the same type as the cathode I3.

The cathodes l3 and 30 are of the filament type having supply leads 45 and 46 which may be connected to any suitable source of A battery potential.

Signals for application to and comparison in the output impedance 25-26 of the discriminator network are derived from the output impedances l8 and I9. On one side, the terminal 2| is connected through a lead 3| and a coupling capacitor 32 to one terminal 33 ofthe output impedance 26. The opposite terminal 34 of the impedance 25 is connected through a similar coupling capacitor 35 with an adjustable signal voltage or gain control device 36 connected in the output circuit 3'! of an amplifier contained in the tube 38 with the diode rectifiers ii and I2 and having the cathode IS in common therewith.

The amplifier anode, indicated at 39, is connected with the output circuit 31 and the control grid 40 is connected through a grid circuit 4| with a grid resistor 42 and'with the terminal 20 at the output impedance I8 through a coupling capacitor 43.

Thus the output impedance 25-26 of the discriminator network is coupled directly with a It will be seen that with the circuit shown,

which provides a balanced discriminator network, amplitude modulation signals, if present, will be balanced out and therefore will not appear in the output circuit 28 for the reason that as a received signal increasesin amplitude the diode rectifier output resistors l8 and I9 will each become more negative at the terminals and 2|. For example, the negative instantaneous portion of the signal voltage at the terminal 2i is applied to, the terminal 33 of the output impedance 2626 while the negative instantaneous portion of the signal voltage at the terminal 28 is applied to the control grid 48 of the amplifier section of the tube 38, causing the anode 39 to become instantaneously more positive. With the device 36 adjusted to provide a gain of substantiall unity through the amplifier between the input ircuit and the terminal 34, the latter terminal will become instantaneously positive to the same degree as the terminal 33 becomes instantaneously negative, and no output voltage will be applied to the output terminal 21, with the resistors or impedances at and 26 substantially equal.

In case a gain of other than unity is desired through the amplifier, the impedances at 25 and 26 may be unequal and thedevice 36 is adjusted to cause a balanced output condition for amplitude modulated signals at the output terminal 21. It should also be noted that the output resistors or impedances at 25 and 26 should be relatively large or high in resistance with respect to the resistance of the output impedances at l8 and IS in the diode rectifier output circuits.

Referring to Fig. 2, wherein the same reference numerals are applied to like circuit elements as in Fig. 1, equipotential cathodes are indicated at 48 and 49 for the tubes 38 and 29, respectively, and a source of bias potential for the tubes is provided in connection with each cathode circuit, as indicated by the self-bias resistors 50 in circuit with the cathodes 48 and 49.

The diode output resistors l8 and I9 are connected to the cathode 48 instead of to ground, and the grid resistor 42 is connected to ground, as indicated at 5|, to derive biasing potential from the resistor 56. The heater circuit for the tubes is indicated at 52 and is of the series type.

Otherwise the circuit of Fig. 2 is the same as that of Fig. 1 and operates in a similar manner. It is shown as an illustration of the method for connecting heater type tubes in the circuit of Fig. 1 at substantially ground cathode potential, since the cathodes are thus operated above ground merely by the D. 0. potential developed in the self-bias resistors 50 and these are each bypassed at'audio frequency potentials by suitable capacitors 55 so that for audio frequency potentials they are at substantially ground potential.

From the foregoing description, it will be seen that the system disclosed is particularly adapted for use in low power and battery operated receivers with filament type cathode! or the like, which may be operated at substantially ground or chassis potential, while permitting a balanced discrimihator circuit effective for eliminating amphtude modulation in a frequency modulation signal receiving system.

I claim as my invention:

1. In a frequency modulation signal receiving system having a grounded cathode circuit, a frequency modulation signal detector including at least one electric discharge rectifier device having a grounded cathode, a frequency discriminator network connected with said rectifier device, a balanced rectifier output circuit balanced with respect to and connected with said grounded cathode in said network, a balanced discriminator output circuit balanced with respect to ground and connected with said balanced rectifier output circuit, and a phase inverter amplifier stage having substantially unity gain interposed in one side of. said last named connection.

2. In a frequency modulation signal receiving system comprising electric discharge tubes 01' the filament type and a grounded filament circuit, a frequency modulation signal detector including at least one electric discharge rectifier device having a grounded filament electrode, a frequency discriminator network connected with said rectifier device, a balanced rectifier output circuit balanced with respect to and connected with said grounded filament in said network, a. balanced discriminator output circuit balanced with respect to ground and connected with said balanced rectifier output circuit, and a phase inverter amplifierstage having substantially unity gain interposed in one side of said last named connection.

3. In a frequency modulation signal detector, the combination of a frequency discriminator network, a pair of rectifier devices connected in balanced relation with said discriminator network, a balanced rectifier output circuit balanced with respect to ground and including an output im-. pedance for each of said rectifier devices having a common grounded cathode connection, a phase inverter amplifier stage, and'a balanced output circuit for said network balanced with respect to ground and having one terminal coupled with one side of said balanced rectifier circuit and having another terminal coupled with the other terminal of said balanced rectifier circuit .through said phase inverter stage.

4. In a frequency modulation signal detector, the combination of a frequency discriminator network,- a pair of rectifier devices connected in balanced relation with said discriminator network, a balanced rectifier output circuit balanced with respect to ground and-including an output impedance for each of said rectifier devices having a common grounded cathode connection, a phase inverter amplifierstage, a balanced output cir-- cuit for said network balanced with respect to ground and having one terminal coupled with one put impedance connected between the low potential terminal of each of said circuits and the cathode ground connection. an amplifier coupled to the high potential terminal of one of said outut impedances, a balanced discriminator out-' put impedance having a signal input terminal coupled to said amplifier and another signal input terminal coupled to the, high potential terminal,

of the other 01' said rectifier output impedances. and means for deriving an output signal from saidoutput impedance including an output circuit connected to an intermediate terminal of said discriminator output impedance.

6. In a frequency modulation signal receiving system-the combination with an intermediate frequency amplifier. of'a frequency modulation detector comprising two intermediate frequency signal circuits tuned above and below a predetermined center frequency, an electric discharge tube having a grounded cathode of the filament type and two anode electrodes associated therewith and connected with the high potential sides of said circuits, 8. rectifier output impedance con"- nected between the low potential side of each of said circuits and around. an electric discharge amplifier having a control grid coupled to the output side of one of said rectifier output impedanees, a discriminator output impedance coupled at one terminal to said amplifier output circuit and coupled at another terminal directly with the output side of the other 01 said rectifier output impedances.

7. In a frequency modulation signal receiving system the combination with an intermediate frequency amplifier, of a frequency modulation detector comprising two intermediate frequency signal circuits tuned above and below a predetermined center frequency; an electric discharge tube having a grounded cathode of the filament type and two anode electrodes associated therewith and connected with the high potential sides of said circuits, a rectifier output impedance connected between the low potential side of each of said circuits and ground, an electric discharge amplifier having a control grid coupled to the output side of one of said rectifier output impedances, an output circuit for said amplifier including output potential adjusting means through said potential adjusting means, and a second electric discharge amplifier having a control grid coupled to an intermediate terminal on said discriminator output impedance.

. wmr'mm a. KOCH. 

